U.S. patent application number 09/911985 was filed with the patent office on 2001-11-22 for dual-mode non-isolated corded system for transportable cordless power tools.
Invention is credited to Carrier, David A..
Application Number | 20010042631 09/911985 |
Document ID | / |
Family ID | 22354014 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010042631 |
Kind Code |
A1 |
Carrier, David A. |
November 22, 2001 |
Dual-mode non-isolated corded system for transportable cordless
power tools
Abstract
A dual-mode system for inexpensively operating electrically
powered double-insulated devices (12), such as hand-held power
tools and appliances. The system includes a cordless battery pack
(14) that supplies the power and current demands of the device (12)
in a cordless mode or a non-isolated corded voltage converter (16)
that supplies the necessary power and current demands in a physical
envelope commensurate in size and interchangeable with that of the
battery pack (14). The corded voltage converter (16) is provided
with a non-isolated high efficiency power supply that allows the
converter (16) to generate the power and current required by the
driven device (12). The double insulation of the driven device (12)
negates the need for a transformer-isolated voltage converter.
Eliminating the power transformer from the converter significantly
reduces the cost of the module (16). Additionally, the need for
multiple battery packs and fast rechargers is minimized by the
availability of a low-cost converter. The voltage converter (16)
includes an inrush current limiter (103) and power conditioner for
filtering AC or DC input power. The filtered voltage is chopped by
a transformerless buck-derived converter. The chopped voltage is
rectified and filtered to provide low-voltage DC power to the drive
motor of the powered double-insulated device (12).
Inventors: |
Carrier, David A.;
(Aberdeen, MD) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, PLC
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
22354014 |
Appl. No.: |
09/911985 |
Filed: |
July 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09911985 |
Jul 24, 2001 |
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09458285 |
Dec 10, 1999 |
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6296065 |
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60114218 |
Dec 30, 1998 |
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Current U.S.
Class: |
173/217 ;
310/50 |
Current CPC
Class: |
H01H 2009/065 20130101;
H02H 7/0852 20130101; H02J 7/0044 20130101; H02J 7/0068 20130101;
B25F 5/02 20130101; H01H 9/06 20130101; H01H 9/52 20130101 |
Class at
Publication: |
173/217 ;
310/50 |
International
Class: |
H02P 007/00 |
Claims
We claim:
1. A corded/cordless system for power tools comprising: a
low-voltage DC power tool operable in a preselected voltage range,
said power tool having an exterior, an interior, a pre-defined
interface for mechanically and electrically mating with a power
supply module, and a double-insulated case to prevent the
conduction of electrical charge from the interior to the exterior
of the power tool; and a corded non-isolated power supply module
mechanically and electrically configured to connect to a source of
relatively high voltage electric power and to mate with the
low-voltage DC power tool, said corded non-isolated power supply
module being adapted to convert said relatively high voltage line
power from the source of electric power to a DC voltage in the
preselected voltage range suitable for powering the low-voltage DC
power tool.
2. The corded/cordless system of claim 1 further comprising: a
cordless battery power supply module mechanically and electrically
configured to mate with the low-voltage DC power tool and to
contain a battery assembly having a DC voltage in the preselected
voltage range suitable for powering the low-voltage DC power tool,
said battery power supply module to provide power from the battery
assembly to the DC power tool; wherein the power tool is configured
to receive power from either the corded non-isolated power supply
module or the battery power supply module.
3. The corded/cordless system of claim 1 wherein the power tool
includes a motor.
4. The corded/cordless system of claim 2 wherein the power tool
pre-defined interface further comprises a physical envelope
configuration to accept either of said corded non-isolated power
supply module and said cordless battery power supply module.
5. The corded/cordless system of claim 4 wherein the power tool
pre-defined interface further comprises an electrical connector
operative to electrically connect to an electrical connector
attached to either of said corded non-isolated power supply module
and said cordless battery power supply module.
6. The corded/cordless system of claim 5 wherein the corded
non-isolated power supply module electrical connector is a terminal
block and the battery power supply module electrical connector is a
terminal block.
7. The corded/cordless system of claim 4 wherein the power tool
pre-defined interface further comprises a latch for releasably
securing either of said corded non-isolated power supply module and
said cordless battery power supply module.
8. The corded/cordless system of claim 1 wherein the corded
non-isolated power supply module comprises: a circuit to convert
power from the source of relatively high voltage electric power to
the DC voltage in the pre-selected voltage range, wherein the DC
voltage is referenced to the power from the source of relatively
high voltage electric power.
9. The corded/cordless system of claim 8 wherein the source of
relatively high voltage electric power provides approximately 120
Vac, 60 Hz power.
10. A corded/cordless system for power tools comprising: a
low-voltage DC power tool operable in a preselected voltage range,
said power tool having an exterior, an interior, a double-insulated
case to prevent the conduction of electrical charge from the
interior to the exterior of the power tool, and a pre-defined
interface for mechanically and electrically mating with a power
supply module; said power tool pre-defined interface comprising: a
physical envelope configuration to accept the power supply module;
a terminal block operative to electrically connect to a terminal
block attached to the power supply module; and a latch for
releasably securing the power supply module; a corded non-isolated
power supply module mechanically and electrically configured to
connect to a source of relatively high voltage electric power and
to mate with the low-voltage DC power tool, said corded
non-isolated power supply module including a circuit to convert
power from the source of relatively high voltage electric power to
a DC voltage in the pre-selected voltage range suitable for
powering the low-voltage DC power tool, wherein the DC voltage is
referenced to the power from the source of relatively high voltage
electric power; a cordless battery power supply module mechanically
and electrically configured to mate with the low-voltage DC power
tool and to contain a battery assembly having a DC voltage in the
preselected voltage range suitable for powering the low-voltage DC
power tool, said battery power supply module to provide power from
the battery assembly to the DC power tool; and wherein the power
tool is configured to receive power from either the corded
non-isolated power supply module or the battery power supply
module.
11. A method of supplying power to a DC power tool operable in a
preselected voltage range, said power tool having exposed surfaces,
the method comprising the steps of: insulating the exposed surfaces
of the DC power tool to prevent the surfaces from becoming
electrically energized; connecting a corded non-isolated power
supply module to the power tool and to a source of relatively high
voltage electric power; converting power from the source of
relatively high voltage electric power to a DC voltage that is not
transformer isolated from the source of relatively high voltage
electric power, wherein said DC voltage is within the pre-selected
voltage range suitable for powering the DC power tool; and powering
the DC power tool with the DC voltage.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of the filing date of
U.S. provisional application No. 60/114218 filed Dec. 30, 1998.
FIELD OF THE INVENTION
[0002] The present invention relates generally to electrically
operated power tools and in particular, to portable hand-held power
tools which can alternatively operate in either a cordless mode
from a self-contained power source or a corded mode from a
conventional AC/DC generator power source.
BACKGROUND OF THE INVENTION
[0003] Electrically operated devices that function in a cordless
mode typically include a housing which has a chamber for receiving
and retaining a removable battery pack. The battery pack completely
encloses one or more cells and provides the necessary DC power for
operation of the device. Historically, cordless electrically
powered devices have included relatively low power devices such as
shavers and hand-held calculators. Recently, improvements in
battery technology have led to the development of batteries that
store more energy and are capable of driving higher power devices.
These devices include for example, portable hand-held power tools
and appliances operating at power levels from 50 watts up to
hundreds of watts. A hand-held power tool is typically powered by a
battery pack that comprises a number of batteries connected in
series. To provide the higher power levels required by high power
devices an increased number of batteries are connected in series
resulting in higher input voltages and battery pack volumetric
requirements.
[0004] Cordless power devices permit work operations to be
performed in areas where a conventional AC power source is not
available or inconvenient to use. However, the effective charge
capacity of the battery pack and the availability of replacement
battery packs limit the use of cordless devices. When the battery
pack is discharged, it must be recharged or replaced with a fully
charged pack.
[0005] Both batteries and battery chargers are expensive in
comparison to the power device for which they are intended.
Batteries for high power applications cost approximately 30% of the
cost of the applicable power device. Additional batteries are
required to permit cordless mode operation while a battery is
recharged and to replace dead batteries. High power levels drawn
from batteries during operation of the power tool, the depth of
discharge of the battery, the number of charge/discharge cycles,
and the speed with which a battery is recharged all contribute to
shortening the usable lifetime of a battery. Fast chargers can cost
more than the power tool or appliance that is powered by the
battery. There are two basic types of battery chargers, trickle
chargers and fast chargers. Trickle chargers are significantly less
expensive than fast chargers, however a trickle charger requires
approximately 1/2 day to recharge a battery pack. A fast charger on
the other hand can recharge a battery pack within approximately one
hour. Therefore, a trade off must be made between using a trickle
charger with a large number of battery packs versus using a costly
fast charger with very few replacement battery packs.
[0006] It has recently been proposed to provide portable cordless
power tools with an optional corded AC converter module that is
connected to an AC power source and designed to replace the battery
pack. The corded converter module converts power from the AC source
to a regulated low-voltage DC level that is usable by the motor of
the power device. Such a device allows a tool operator to use the
tool in either the cordless battery mode or the corded AC mode as
needed. Thus, the availability of such device enables the operator
of a cordless tool to complete a project when the battery pack has
been discharged, or to continue to use the tool while the battery
pack is charging and a fully charged backup battery pack is
unavailable. Hence, by using a corded converter module the need for
extra battery packs is minimized.
[0007] However, the prior art design of a corded converter module
is constrained by a number of factors such as the physical
envelope, the required output power level, the voltage conversion
ratio of the converter, safety requirements to protect the operator
from electrical shock, and cost. The envelope of the corded
converter module must conform to the envelope of the battery pack
with which it is interchangeable. With the increased volumetric
requirements for battery packs there is increased volume available
for housing a corded converter. The power output level of the
converter is directly related to the available volume within the
container envelope. The power output levels adequate to drive power
devices such as hand held power tools are possible within the
physical envelope of commercial battery packs. The voltage
conversion ratio of the converter is the ratio between the
rectified input voltage and the converter output voltage. The
converter output voltage is set to a level roughly equivalent to
the battery voltage. The greater the voltage conversion ratio the
more difficult it is to accurately regulate the output voltage. The
safety regulations are typically met by isolating the operator of
the power device from the AC power source. Commercially available
systems meet the safety regulations by employing a high frequency
power transformer to isolate the output power of the converter
module from the relatively high voltage AC input power source.
Power transformers are custom devices that are expensive and bulky
in comparison with the other electronic devices of the converter
module. Attempts to minimize costs of corded converter modules have
concentrated on optimizing the output power capability of the
converter module for a given power device. By designing the
converter module for the minimum output power required to
satisfactorily drive the power device, lower cost electronic
components can be chosen for the converter.
[0008] Operators of cordless power tools already faced with the
cost of battery packs and battery chargers must also invest in
expensive corded converter modules for their power tools. As an
alternative many purchase a corded power tool to use in lieu of the
cordless tool when an AC power source is nearby. Attempts to
minimize the cost of corded conversion modules have been
constrained by the cost of using transformer isolation to meet the
government safety requirements. Obtaining further cost reductions
by reducing the output power level of a corded converter module
would result in under-powered power devices. While the prior art
can be used to provide corded converter modules for a handheld
power tool, it has not proven capable of providing low cost modules
that are convenient to use.
SUMMARY OF THE INVENTION
[0009] The present invention decreases costs by meeting the
government safety requirements in a unique manner. The invention
uses a double insulated casing for the power tool rather than
employing transformer isolation. Eliminating the power transformer
from the corded converter module significantly reduces the cost and
weight of the module. A low cost converter module provides
operators of cordless power tools the low cost option of using a
corded converter module when AC power sources are available. This
eliminates the cost of purchasing a separate corded power device as
well as reducing the number of battery packs that must be
purchased.
[0010] Corded power converters designed without power transformers
are substantially less expensive than converters designed with
power transformers. Additionally, eliminating the power transformer
decreases the weight of the converter resulting in improved
operator comfort.
[0011] For a more complete understanding of the invention, its
objects and advantages, reference may be had to the following
specification and to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a three-dimensional view partially showing the
manner of connecting a battery pack to the power device;
[0013] FIG. 2 is a three-dimensional view partially showing the
manner of connecting an AC/DC power converter module to the power
device;
[0014] FIG. 3A is a three-dimensional exploded view of the battery
pack;
[0015] FIG. 3B is a three-dimensional exploded view of the power
converter module;
[0016] FIG. 4 is an end view of the battery pack illustrating an
attached terminal block;
[0017] FIG. 5 is a three-dimensional view of the power tool
terminal block that mates to the battery pack terminal block;
[0018] FIG. 6 is a two-dimensional view of the interface between
the battery pack terminal block and the power tool terminal
block;
[0019] FIG. 7 is a two-dimensional view of the interface between
the AC/DC power converter module and the power tool terminal
block;
[0020] FIG. 8 is a block diagram of a power converter assembled and
contained within the AC/DC power converter module of FIG. 2;
[0021] FIG. 9 is a schematic diagram of the power stage of the
power converter of FIG. 8;
[0022] FIG. 10 is a schematic diagram of the control circuit of the
power converter of FIG. 8;
[0023] FIG. 11 is a signal diagram showing the voltage and current
waveforms associated with the power converter;
[0024] FIG. 12 is a cross-sectional view of an armature of a
non-double insulated DC power tool motor;
[0025] FIG. 13 is a cross-sectional view of an armature of DC power
tool motor that employs a first method of double insulation;
[0026] FIG. 14 is a cross-sectional view of an armature of DC power
tool motor that employs a second method of double insulation;
[0027] FIG. 15 is a cross-sectional view of an armature of DC power
tool motor that employs a third method of double insulation;
[0028] FIG. 16 is cross section through the center of the
lamination stack of an armature for a DC power tool motor that
employs double insulation; and
[0029] FIG. 17 is a cross-sectional view of a housing for a DC
power tool that employs double insulation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] Referring to FIGS. 1 and 2, a dual-mode portable power tool
12 according to the present invention is shown. While the present
invention is shown and described with a reciprocating saw 12, it
will be appreciated that the particular tool is merely exemplary
and could be a circular saw, a drill, or any other similar portable
power tool constructed in accordance with the teachings of the
present invention.
[0031] The power tool 12 includes a DC motor (not shown) that is
adapted in the preferred embodiment to be powered by a source
having a relatively low voltage such as a 24 volt DC source,
although other low voltage DC systems, such as 12 volts or 18
volts, could be used. In a first operating mode shown in FIG. 1,
the power tool 12 is powered by a removable battery power supply
module 14. Alternatively, as shown in FIG. 2, the power tool 12 may
be powered from a source having a relatively high voltage such as
common 115 volt AC line power via an AC/DC power converter module
16 which is adapted to be plugged into the power tool in place of
the battery power supply module 14. Additionally, the power tool 12
may be powered from a relatively high voltage DC generator (not
shown) via the AC/DC power converter module 16. As used in this
specification and the accompanying claims, the term relatively high
voltage means voltages of 40 volts or greater and the term
relatively low voltage means voltages less than 40 volts.
[0032] With particular reference to FIGS. 3A and 4, the
rechargeable battery power supply module 14 of the present
invention is illustrated to generally include a housing 18, a
battery 20 which in the exemplary embodiment illustrated is a 24
volt nickel-cadmium battery, and a battery pack terminal block 22.
To facilitate releasable attachment of the battery power supply
module 14 to the tool 12, the upper portion 25 of the housing 18 is
formed to include a pair of guide rails 24. The guide rails 24 are
adapted to be slidably received into cooperating channels 13 (FIG.
1) formed in a housing 14 of the tool 12. To further facilitate
removable attachment of the battery power supply module 14 to the
tool 12, the upper portion 25 of the housing 18 further defines a
recess 26. The recess 26 is adapted to receive a latch (not shown)
carried by the housing of the tool 12. The latch is conventional in
construction and operation and is spring biased to a downward
position so as to engage the recess 26 upon insertion of the
rechargeable battery power supply module 14. Removal of the battery
power supply module 14 is thereby prevented until the spring bias
of the latch is overcome in a conventional manner insofar as the
present invention is concerned.
[0033] With continued reference to FIGS. 3A and 4, the battery pack
terminal block 22 comprises a main body portion 28 constructed of
rigid plastic or other suitable material and a plurality of
blade-type terminals 30. In the exemplary embodiment illustrated,
the battery pack terminal block 22 includes four blade terminals
30. Two of the blade terminals 30 comprise the positive and
negative terminals for the battery 20. A third terminal 30 may be
used to monitor the temperature of the battery 20 and a fourth
terminal may be used to identify the battery type (e.g., 24 volt
NiCad). As best shown in FIG. 4, a pair of holes 32 are formed in
the two guide rails 24 in the upper portion 25 of the battery pack
housing 18 on either side of the row of blade terminals 30. The
function of these holes is described below.
[0034] Turning now to FIG. 5, the terminal block 34 of the power
tool 12 is shown. The main body of the tool terminal block 34 is
also constructed of a rigid plastic material and is formed with a
row of four U-shaped guideways 36 guiding the four corresponding
blade terminals 30 of the battery power supply module 14 when the
battery pack is inserted into the tool 12. Located within the
guideways 36 are female connectors 38 that are adapted to engage
and make electrical contact with the blade terminals 30 of the
battery power supply module 14. Although the tool terminal block 34
shown is designed to accommodate four female connectors for each of
the four battery pack blade terminals 30, only two female
connectors 38 adapted to engage the positive and negative blade
terminals 30 of the battery power supply module 14 are used in the
tool terminal block 34, as the remaining two battery pack blade
terminals 30 are only used when recharging the battery power supply
module 14.
[0035] Also connected to the positive and negative female terminals
38 in the tool terminal block 34 are positive and negative male
terminals 40 that project through openings 42 in the terminal block
on either side of the row of guideways 36. As will subsequently be
discussed below, the male positive and negative terminals 40 are
used to electrically connect the tool 12 to the AC/DC converter
module 16.
[0036] With additional reference to FIG. 6, the interface between
the battery terminal block 22 and the tool terminal block 34 is
illustrated. As the guide rails 24 of the battery power supply
module 14 are slid into the channels 13 in the tool housing, the
battery pack terminal block 22 is guided into alignment with the
tool terminal block 34 as shown. To further facilitate proper
alignment between the two terminal blocks 22 and 34, the main body
portion of the tool terminal block 34 includes a pair of laterally
spaced rails 44 that are adapted to be received within the grooves
46 provided in the battery pack housing 18 immediately below the
guide rails 24. Further insertion of the battery power supply
module 14 onto the tool 12 results in the positive and negative
blade terminals 30 of the battery power supply module 14 passing
through the openings in the U-shaped guideways 36 and engaging the
female connectors 38 in the tool terminal block 34. Note that the
male positive and negative terminals 40 from the tool terminal
block 34 simultaneously project into the openings 32 formed in the
rails 24 on the upper portion 25 of the battery pack housing 18,
but do not make electrical contact with any terminals in the
battery power supply module 14. Similarly, the remaining two blade
terminals 30 from the battery terminal block 22 project into empty
guideways 36 in the tool terminal block 34.
[0037] Returning to FIG. 2 with reference to FIG. 3B, the AC/DC
converter module 16 according to the present invention is adapted
to convert 115 volts AC house current to 24 volts DC. The housing
48 of the converter module 16 in the preferred embodiment is
configured to be substantially similar to the housing 18 of the
battery power supply module 14. In this regard, the housing 48
includes first and second clam shell halves joined at a
longitudinally extending parting line. An upper portion 50 of the
housing 48 includes a pair of guide rails 52 similar to those of
the battery power supply module 14 for engaging the channels 13 in
the tool housing. The upper portion 50 also defines a recess (not
shown) which includes a latch (not shown) for preventing the
inadvertent removal of the converter module 16. The housing 48 also
defines a recess 51 in which a fan 45 is adapted for providing
cooling airflow to the converter module 16. Attached to the fan 45
is a fan cover 47 for preventing foreign objects from impeding the
operation of the fan 45. Within the housing 48 several heatsinks 43
provide heat spreading and cooling for selected power converter
components.
[0038] With additional reference to FIG. 7, the interface between
the converter module 16 and tool terminal block 22 is shown. The
converter module 16 includes a pair of female terminals 54 that are
adapted to receive the male terminals 40 of the tool terminal block
22. In a manner similar to that described above in connection with
the installation of the battery power supply module 14 on the tool
12, the guide rails 52 on the upper portion 50 of the converter
housing 48 are adapted to engage the laterally spaced rails 44 on
the tool terminal block 34 as the converter module 16 is installed
on the tool 12 to ensure proper alignment between the female
connectors 54 of the converter module 16 and the male connectors 40
of the tool 12.
[0039] Due to the non-isolated nature of the AC/DC converter module
16 in the present invention, the female terminals 54 are recessed
within the upper portion 50 of the housing 48 of the converter
module 16 to meet safety requirements. In the preferred embodiment,
the female terminals 54 are recessed within the housing 48 of the
converter module 16 by at least 8 mm. 115 volt AC power is
converted to 24 volt DC power by the converter module 16 and
delivered to the tool 12 through the female terminals 54. When the
converter module 16 is operatively installed on the tool 12, the
female terminals 38 of the tool terminal block 34 are electrically
inoperative.
[0040] The presently preferred embodiment of the AC/DC power
converter module 16 is a fixed-frequency, non-isolated,
buck-derived topology; however, the principles of the invention can
be extended to variable-frequency converters and topologies other
than buck-derived, such as Cuk and flyback converters. The power
converter module 16 is designed to convert an unregulated AC
voltage to a regulated DC voltage that is usable by the power tool
12. For example, the converter module 16 can convert an input of
120 volts, 60 Hz AC to any low-level DC voltage less than 42 volts
that is required by the power tool 12, such as 24 volts DC.
[0041] As illustrated in block diagram form in FIG. 8, the power
converter module 16 includes a fuse 101 in series with diode bridge
102. A power plug and cord (refer to FIG. 2) connect from fuse 101
to the other input of diode bridge 102. The output of diode bridge
102 is applied between high side line 104 and an inrush limiter 103
connected to ground reference line 106. The rectified output
voltage of diode bridge 102 is filtered by the input capacitor 108.
The resulting filtered voltage is nominally 165 volts DC. The input
capacitor 108 connects to the drains of parallel power MOSFETs 110a
and 110b that act as a voltage controlled switch. When MOSFETs 110a
and 110b are in the ON state the impedance between the drain and
source is low. When in the OFF state the impedance between drain
and source is very high, effectively preventing current flow. The
sources of MOSFETs 110a and 110b connect to the junction of output
inductor 112 and the cathode of free-wheeling output diode 114. The
other side of output inductor 112 connects to output capacitor 116.
Current sense resistor 118 connects between the output capacitor
116 and the anode of the freewheeling diode 114. The anode of
output diode 114 also connects to ground reference line 106. The
voltage across output capacitor 116 is applied to the output of
power converter module 16 across outputs VOUTHI 120 and VOUTLO 122,
which connect to the pair of female terminals 54. Fan 123 is
connected in parallel with output capacitor 116. Diode bridge 102,
MOSFET 110, and free-wheeling output diode 114 all mount on heat
sinks that provide heat spreading and a thermal path for dissipated
power.
[0042] FIGS. 8 and 10 illustrate the circuitry that provides
control and protection functions for power converter module 16
which includes voltage regulated power supply 124, PWM control 126,
voltage feedback 128, current limit 130, and temperature sense 134.
The voltage regulated power supply 124 connects across input
capacitor 108 to provide a low power, regulated low voltage output
to supply power to the internal circuitry of power converter module
16. The regulated low voltage output as well as the remainder of
the internal circuitry is referenced to ground reference line 106.
VOUTHI 120 connects to voltage feedback 128 which connects to PWM
control 126. The current sense resistor 118 connects to current
limit 130 which also is connected to temperature sense 134. The
output of current limit 130 connects to PWM control 126. The
arrangement of components that comprise voltage regulated power
supply 124, PWM control 126, voltage feedback 128, current limit
130, and temperature sense 134 are well known in the art.
[0043] FIGS. 9 and 10 illustrate the circuitry that provides the
power conversion function for power converter module 16 which
includes high voltage driver 132 and power stage components. The
output of PWM control 126 connects to high voltage driver 132 which
level shifts the output of PWM control 126 to drive the gates of
MOSFETs 110a and 110b. The arrangement of components that comprise
high voltage driver 132 are well known in the art. In the presently
preferred embodiment of the invention an SGS-Thomson L6381
high-side driver 172 with associated components comprises the high
voltage driver 132. However, other circuit configurations for
level-shifting the PWM output are within the scope of the
invention, such as discrete component configurations and Motorola
high-side driver chips.
[0044] Referring to FIG. 8, at initial power-on of power converter
module 16, the power plug and cord are connected to an AC power
source. The AC voltage is rectified by diode bridge 102 and applied
across input capacitor 108. Current from the AC source surges as it
flows through fuse 101, inrush limiter 103, diode bridge 102, and
begins to charge input capacitor 108. The magnitude of the surge in
current is limited to a safe level by the action of the inrush
limiter 103 which is a high impedance initially, but rapidly
changes to a low impedance. In the present embodiment the inrush
limiter 103 consists of a triac 152 in parallel with a resistor 150
that is triggered by current flowing through output inductor 112.
However, other well known circuits are also envisioned, such as a
series thermistor, and a high valued series resistor in parallel
with a controlled semiconductor that is triggered by temperature,
time, or current magnitude. As the voltage across input capacitor
108 rises towards its nominal value of 165 volts DC the voltage
regulated power supply 124 becomes active and begins to supply
voltage to the internal circuitry of the power converter module 16
including PWM control 126. During the initial charging of input
capacitor 108, the triac 152 remains off forcing return current to
flow through resistor 150, thereby limiting the peak value of the
inrushing current. The triac 152 remains OFF until the output of
PWM control 126 becomes active driving the MOSFETs 110a and 110b to
the ON state, at which time current flowing through output inductor
112 couples through a sense winding of inductor 112 to trigger the
triac ON.
[0045] The PWM control 126 in the present embodiment is a Texas
Instruments TL494 with the associated components as depicted in
FIG. 10. There are numerous other control chips which could be
used, such as UC1845 and SG1625. The output of PWM control 126 is
disabled until the regulated output of voltage regulated power
supply 124 exceeds 6.4 volts, at which time soft-start mode is
enabled. Prior to the beginning of soft-start the oscillator of PWM
control 126 begins to operate. The present embodiment switches at a
fixed frequency of 40 kHz, although higher or lower frequencies are
within the scope of the invention. During steady-state operation of
power converter module 16 the PWM control 126 output is a
low-voltage square-wave signal having a variable pulse-width, where
the pulse-width is adjusted to maintain a regulated output voltage
at outputs VOUTHI 120 and VOUTLO 122. During soft-start the
pulse-width of the PWM control 126 output is initially zero,
gradually increasing to a steady-state value that results in the
output voltage being regulated at a desired voltage. The duration
of soft-start mode is controlled by the selection of component
values in PWM control 126 and is well known in the art. The purpose
of soft-start is to limit the current and voltage stress of the
power converter module 16 components during the time period when
output capacitor 116 is being charged up to its nominal
steady-state value. As the voltage across output capacitor 116
approaches its steady-state value the output of voltage feedback
128 rises towards its steady-state value, resulting in the
pulse-width of PWM control 126 attaining a steady-value that
regulates the voltage across output capacitor 116 at the desired
value. The feedback network in the present embodiment is a
lag-lead-lag-lead configuration with well known design requirements
to maintain a stable operation of power converter module 16. During
steady-state operation the output from PWM control 126 which is
level-shifted by the high voltage driver 132 repetitively drives
the MOSFETs 110a and 110b into an ON state and an OFF state at the
switching frequency.
[0046] Referring to waveforms vs, iL, and vout of FIG. 11 in
addition to FIG. 8, when MOSFETs 110a and 110b are in the ON state,
the voltage from input capacitor 108 is passed through to the
sources of MOSFET 110a and 110b, vs, and impressed on the input of
output inductor 112 reverse biasing free-wheeling diode 114. The
voltage across output inductor 112 during the ON state is equal to
the voltage across input capacitor 108 minus the voltage across
output capacitor 116, vout. The positive voltage across inductor
112 causes current, iL, through inductor 112 to increase at a
linear rate. The current splits between VOUTHI 120 and output
capacitor 116 with the DC component flowing to VOUTHI 120 and the
AC component substantially flowing through output capacitor 116.
Current returning from load 121 flows from VOUTLO 122 through
current sensor resistor 118 and input capacitor 108 thereby
completing the current path.
[0047] When the MOSFETs 110a and 110b are switched to the OFF
state, they present a high impedance to the voltage from input
capacitor 108 decoupling that voltage from the remainder of the
circuit. During this period free-wheeling diode 114 is active. The
current, iL, from output inductor 112 which previously flowed
through MOSFETs 110a and 110b now flows through free-wheeling
output diode 114. With output diode 114 conducting, the voltage,
vs, at the input to output inductor 112 is approximately one diode
drop below ground reference line 106. The voltage across output
inductor 112 is equal to negative one volt minus the voltage across
output capacitor 116. The negative voltage across inductor 112
causes current through inductor 112 to decrease at a linear rate.
The current again splits between VOUTHI 120 and output capacitor
116 with the DC component flowing through VOUTHI 120 and the AC
component substantially flowing through output capacitor 116. The
current returning from load 121 flows from VOUTLO 122 through
current sense resistor 118 and free-wheeling output diode 114,
thereby completing the current path. The MOSFETs 110a and 110b
remain in the OFF state for the remainder of the cycle time
period.
[0048] Again referring to FIG. 8 with additional reference to
waveforms vg and vpwm of FIG. 11, the output of PWM control 126 is
level-shifted by high voltage driver 132 in order to drive power
MOSFETs 110a and 110b to either the ON state or the OFF state.
During the transition from the OFF state to the ON state, the PWM
control 126 output voltage, vpwm, transitions low which causes the
output of driver 172 to transition high, thus biasing the base
emitter junction of PNP transistor 178 turning it OFF. At the same
time NPN transistor 174 turns ON. Current flows through NPN
transistor 174 and resistors 176a and 176b into the gates of power
MOSFETs 110a and 110b charging up the internal gate-source
capacitance, raising the MOSFETs 110a and 110b gate voltage, vg,
above ground before returning from the sources of MOSFETs 110a and
110b to filter capacitor 168. The increasing voltage across the
gate-source of MOSFETs 110a and 110b causes the MOSFETs 110a and
110b to begin to turn ON, causing the source voltage of MOSFETs
110a and 110b to increase from minus one volt relative to ground
reference line 106 to a value approaching the value of voltage
across input capacitor 108 and additionally causing the MOSFETs
110a and 110b gate voltage, vg, to increase to the value of voltage
across input capacitor 108 plus the MOSFETs gate-source voltage. As
the source voltage of MOSFETs 110a and 110b increases, the
decoupling diode 166 becomes reverse biased decoupling the diode
166 from the remainder of the high voltage driver 132. Filter
capacitor 168 remains referenced to the source of MOSFETs 110a and
110b and thereby provides the energy required to maintain the
gate-source voltage of MOSFETs 110a and 110b during the remainder
of the ON state.
[0049] The PWM control 126 output voltage, vpwm, transitions from a
low to a high value to initiate the start of the OFF state. The
high-side driver 172 inverts and level shifts the signal which
causes NPN transistor 174 to turn OFF and PNP transistor 178 to
turn ON. The energy stored in the internal gate-source capacitance
of MOSFETs 110a and 110b discharges through resistor 176 and PNP
transistor 178. When the gate-source voltage of MOSFETs 110a and
110b decreases to less than approximately four volts MOSFETs 110a
and 110b turn OFF. Free-wheeling diode 114 becomes active which
causes the voltage at the sources of MOSFETs 110a and 110b to
decrease to minus one volt. Current then flows through decoupling
diode 166 into filter capacitor 168 recharging the capacitor 168.
Parallel zener diode 170 clamps the voltage across filter capacitor
168 to a safe value that does not overstress the gate-source
junctions of the MOSFETs 110a and 110b. The circuit remains in the
OFF state until the output of PWM control 126 once again
transitions low.
[0050] In addition to controlling pulse width to maintain a
constant output voltage, PWM control 126 also varies the pulse
width in response to an output from current limit 130 to protect
power converter module 16 from excessive output current loads.
Output current flows through current sense resistor 118 causing a
voltage to develop that is proportional to the output current. The
voltage across resister 118 is compared to a reference voltage
derived from the PWM control reference. When the output current is
greater than a pre-determined maximum level the output of current
limit 130 causes PWM control 126 to reduce the pulse width of the
output. The reduced duty cycle causes the voltage at outputs VOUTHI
120 and VOUTLO 122 to decrease until the resulting output current
is less than the pre-determined maximum level.
[0051] Temperature sense 134 protects power converter module 16
from overtemperature stress of MOSFET 110 and output diode 114. In
the presently preferred embodiment a thermistor is employed as
temperature sense 134 to monitor the temperature of heatsinks 43.
If the temperature rises due to overload, debris blocking an air
intake, or other fault condition, temperature sense 134 modifies
the current limit reference voltage, thereby causing the PWM
control 126 to generate a shorter pulse width. The shorter pulse
width results in a lower output voltage and output current that
corresponds to a lower overall output power. The lower output power
causes a reduction in the power dissipated in the components of
power converter module 16, resulting in lower component
temperatures.
[0052] Returning to FIG. 1, although the power tool 12 of the
present invention is designed to be powered by a relatively low
voltage DC power source (i.e., a DC source less than 50 volts), the
housing 201 of the power tool in the preferred embodiment is
nonetheless double insulated from the electrical system of the
tool. As is well known to those skilled in the art, power tools
designed to be operated by a high voltage power source, such as a
conventional AC or corded power tool, are typically constructed so
that the housing of the tool is double insulated from the
electrical system of the tool for safety reasons. In this manner,
the operator of the tool is protected against electrical shock in
the event of a short in the electrical system of the tool. Cordless
or DC powered tools are powered by low voltage power sources and
therefore do not require such safety measures. Consequently,
conventional DC powered tools do not insulate the housing from the
electrical system of the tool.
[0053] There are of course, many DC powered portable devices that
are alternatively powered from high voltage AC house current. To
enable this alternative operation, however, AC/DC powered devices
universally employ transformers to step down the high AC voltage
and thereby isolate the device from the high voltage AC power
source.
[0054] While this solution may be acceptable for relatively low
powered devices, such as portable stereos, the power requirements
of many portable power tools necessitates the use of large
step-down transformers which are not only bulky, but also very
heavy. Consequently, DC powered tools that can alternatively be
powered from AC house current have rarely been offered
commercially.
[0055] The present invention solves this dilemma by providing a
relatively light weight non-isolated AC to DC converter and then
constructing the DC powered tool in a manner consistent with the
double insulation safety requirements of a conventional AC powered
tool. In other words, by eliminating transformer isolation in the
present AC/DC power converter module 16, the DC output voltage
supplied to the motor of the power tool is referenced to the 115
volt AC input. Consequently, double insulation of the tool housing
from the electrical system of the power tool is required.
[0056] In addition, as discussed above in connection with the
description of FIGS. 5-7, the power tool terminal block 34
according to the present invention is provided with independent
male connectors 40 uniquely adapted to make electrical contact
with, and thereby receive electrical power from, specially recessed
female connectors 54 in the AC/DC converter module 16. Thus,
despite the non-isolated construction of the present AC/DC
converter module 16, all applicable safety requirements for
operating a power tool from a relatively high voltage power source
are satisfied.
[0057] FIGS. 12 through 17 depict the effect of employing double
insulation within a motor and housing. Double insulation techniques
are well known in the art. Double insulated tools are typically
constructed of two separate layers of electrical insulation or one
double thickness of insulation between the operator and the tool's
electrical system. With specific reference to FIG. 12, a
cross-sectional view of a non-double insulated DC motor armature
200 is illustrated. The armature 200 consists of a shaft 202 with a
core built up over it. The core is composed of many laminations 206
with notches along the outer periphery to hold the armature
windings 204. A gear or chuck (not shown) is built onto the shaft
at one end of the armature 206 to provide a means of transferring
rotational energy to the working end 208 (see FIG. 1) of the power
tool 12. For example a gear mechanism would convert rotational
energy to the forward and back motion used to drive a reciprocating
saw. The path from the armature shaft 202 to the gear mechanism or
chuck, and finally to the working end is electrically conductive.
Therefore any electrical energy that exists on the armature shaft
202 is conducted to the working end, which is exposed to the
operator of the power tool 12. Locations 208, 210, and 212 indicate
areas of the rotor that could become energized through contact with
electrically live assemblies if insulation is not employed. At
location 208 the armature shaft 202 could be energized through
contact with energized armature laminations 206. At location 210
the armature shaft 202 could be energized through contact with end
turns of the armature windings 204. At location 212 the armature
laminations 206 could be energized through contact to end turns of
the armature windings 204.
[0058] Referring to FIG. 13, a first method of employing double
insulation of the motor armature 220 of a power tool is
illustrated. The armature 220 consists of a shaft 222 with a core
built up over it. The core is composed of many laminations 226 with
notches along the outer periphery to hold the armature windings
224. A chuck 228 is built onto the shaft at one end of the armature
laminations 206 to provide a means of affixing a device such as a
drill bit to the working end 208 (see FIG. 1) of the power tool 12.
A molded plastic insulator 230 provides basic insulation between
the armature windings 224 and the laminations 226 as well as
between the shaft 222 and the windings 224. A press fit plastic
tube insulator 232 encases the shaft 222 providing supplementary
insulation to prevent the shaft from becoming energized if the
basic insulation breaks down.
[0059] Referring to FIG. 14, a second method of employing double
insulation of the motor armature 220 of a power tool is
illustrated. A paper insulator 240 provides basic insulation
between the armature windings 224 and the laminations 226. A second
insulator 242 of double thickness, 2 mm, encases the shaft 222
providing reinforced insulation, which substitutes for
supplementary insulation, to prevent the shaft from becoming
energized through electrical shorts to the laminations 226 or the
armature windings 224.
[0060] Referring to FIG. 15, a third method of employing double
insulation of the motor armature 220 of a power tool is
illustrated. An insulator 250 of either paper or molded plastic
provides basic insulation between the armature windings 224 and the
laminations 226. An in situ molded thermoset plastic insulator 252
of double thickness encases the shaft 222 providing reinforced
insulation, which substitutes for supplementary insulation, to
prevent the shaft from becoming energized through electrical shorts
to the laminations 226 or the armature windings 224.
[0061] Referring to FIG. 16, a cross-section through the center of
the lamination stack of the motor armature 220 of a power tool is
illustrated. A slot liner insulator 260 provides basic insulation
between the armature windings 224 and the laminations 226. The slot
liner insulator is constructed of any suitable electrical insulator
material such as paper, coated paper, polyester, and vulcanized
fiber. Supplementary insulation is provided by a glass reinforced
polyester insulator sleeve 262 which encases the shaft 222. The
insulator sleeve prevents the shaft from becoming energized if the
basic insulation provided by slot liner 260 fails.
[0062] Referring to FIG. 17, a double insulated housing 270 of a
power tool is illustrated. As is known in the art, the double
insulation methods employed are intended to prevent electrical
energy within the housing 270 from energizing the outside surface
of the housing 270. The housing 270 is depicted with a hypothetical
metal foil covering 272 on the outside surface to simulate
interaction with an operator. Also illustrated are a ring terminal
274 and an insulated wire 276 that includes a conductive wire 278
and wire insulation 280. Electrical energy exists on both the ring
terminal 274 and the conductive wire 278. Double insulation of the
ring terminal 274 is provided by a double thickness, 2 mm, of
housing material which serves as a reinforced insulator. The wire
insulation 280 provides basic insulation for conductive wire 278.
Supplementary insulation is provided by the housing 270 which
prevents electrical energy that breaks through the wire insulation
from energizing the outside surface of the housing 270.
[0063] The power converter module 16 initially converts the low
frequency AC input to a high level DC voltage, then to a high
frequency voltage level that is thereafter filtered to the lower
voltage supply level of power tool 12. The power tool employs
double insulation of the motor rather than transformer isolation of
the power converter 16, thereby significantly reducing the cost and
weight of the power converter module 16.
[0064] In addition, the converter module 16 is designed with a
comparatively small number of components while providing an
efficient conversion process. This further enhances the
lightweight, compact features of the converter module 16. The size
of the converter module 16 further permits the use of the converter
in power-operated devices, such as the reciprocating saw 12, which
heretofore were too small to support and contain conversion units
providing power in a range of at least 50 watts and higher.
[0065] Further, while the preferred embodiment of the converter
module 16 converts a low frequency, high voltage level to a low DC
voltage level, the converter can be used to convert a high DC
voltage level to a low voltage DC level by applying the high DC
level directly to a suitable power cord and plug that connects to
the input of converter module 16. In this manner, the power tool 12
could be operated from the high DC voltage source instead of the
low DC voltage of the cells 26 and thereby conserve the charge life
of the cells.
[0066] The converter module 16 could be designed to operate from
external AC power sources other than 120 volts at 60 Hz. Without
departing from the spirit and scope of the invention, the converter
module 16 also could be designed to provide DC output voltage
levels in a range of 3.6 to 48 volts. In a particular example, the
converter could be adjusted to develop a DC output of 24 volts
between the outputs VOUTHI 120 and VOUTLO 122 derived from an
external AC source of 220 volts at 50 Hz as applied to a suitable
power plug and cord. The converter module 16 could then be used to
provide inexpensive dual mode capability for power-operated devices
that operate at a DC voltage supply level of 24 volts.
[0067] The reciprocating saw 12 is merely illustrative of one
example of many power-operated, cordless-mode devices that become
more versatile because of the inventive cost efficient dual-mode
capability. Other examples of power-operated cordless devices which
are enhanced by the inventive concept include, but are not limited
to, drills, screwdrivers, screwdriver-drills, hammer drills, jig
saws, circular saws, hedge trimmers, grass shears, as well as
battery-operated household products and the like.
[0068] Thus it will be appreciated from the above that as a result
of the present invention, an inexpensive dual-mode corded/cordless
system for power-operated devices is provided by which the
principal objectives, among others, are completely fulfilled. It
will be equally apparent and is contemplated that modification
and/or changes may be made in the illustrated embodiment without
departure from the invention. Accordingly, it is expressly intended
that the foregoing description and accompanying drawings are
illustrative of preferred embodiments only, not limiting, and that
the true spirit and scope of the present invention will be
determined by reference to the appended claims and their legal
equivalent.
* * * * *